1. Field of the Invention
The present invention relates to a technology for feeding sheets in an image forming apparatus.
2. Description of the Related Art
In typical image forming apparatuses, a sheet feeding device employing a friction pad separation method is used, in which recording sheets or originals are separated one by one for sheet feeding. Such sheet feeding device employing the friction pad separation method includes a rotatable sheet loading board and a friction-member supporting member. The sheet loading board is disposed such that an uppermost sheet among stacked sheets is biased towards a pickup roller or a sheet feeding roller by using a pressing unit such as a spring. The friction-member supporting member similarly biases a friction pad (a friction member) towards the sheet feeding roller by using the pressing unit and separates sheets that have been fed towards the sheet feeding roller from the sheet loading board one by one by using the friction member. At that time, if a pressure (hereinafter, “sheet feeding pressure”) of the sheet loading board is too high, a supply capability of the sheets increases. Therefore, the sheets are not separated by the friction member and double sheet feeding occurs. If the sheet feeding force is too low, the supply capability of the sheets decreases than a conveyance load due to the friction member etc., and sheet feeding fails. On the other hand, when a pressure (hereinafter, “separation pressure”) of the friction-member supporting member is too low, a frictional force sufficient for sheet separation is not obtained, and double sheet feeding occurs. If the separation force is too high, abnormal noise is generated due to stick-slip (a phenomenon involving a minute repetition of an operation in which a sheet is momentarily stopped on the friction pad and then conveyed) between the friction member and a sheet. To prevent such problems, the sheet feeding pressure and the separation pressure need to be set in an appropriate level.
The appropriate level for the sheet feeding pressure and the separation pressure differs according to a sheet type such as thick sheets and thin sheets. Normally, because thick sheets are hard, conveyance load increases and a feeding failure is likely to occur. On the other hand, thin sheets are likely to cause double sheet feeding. To make the sheet feeding device compatible with various sheet types, it is necessary to set the sheet feeding pressure and the separation pressure in a common appropriate level that can support various sheet types. However, if a width corresponding to a sheet weight of the sheet feeding device is wide, the common appropriate level does not exist.
In a technology disclosed in Japanese Patent Application Laid-open No. 2004-189350, even when a length of the sheet feeding roller cannot be adequately secured, squealing (abnormal noise), double sheet feeding, feeding failure occurring in the friction-pad separation method can be controlled. In the above technology, a central portion of a friction pad is disposed opposite to the sheet feeding roller and both side portions of the friction pad are disposed on both sides of the sheet feeding roller. Thus, a level difference is created by sinking the central portion of the friction pad with respect to both side portions. Moreover, a magnitude relation for the width is set such that the width of the sheet feeding roller is thinner than the width of the central portion of the pad and the recording media are compressed into a depression of the friction pad by using the sheet feeding roller.
In a technology disclosed in Japanese Patent Application Laid-open No. 2005-343582, a simple structure is suggested in which sheets do not rub against left and right edges of the friction pad and the sheets are not damaged or curled. For that, an inclined guiding unit is set at a portion of the friction pad that carries sheets, and the friction pad is disposed downstream of a sheet conveyance direction of the guiding unit. Further, ribs that can move higher than an upper surface of the friction pad are vertically arranged on both sides of a rear end of the disposed portion of the friction pad with respect to the sheet conveyance direction. Moreover, the sheet feeding roller is disposed on a peripheral central portion of a sheet feeding collar set concentrically with a shaft and narrow pressure rollers are arranged on both end portions of the sheet feeding roller. A diameter of the pressure roller is set slightly shorter than the diameter of the sheet feeding roller, and both side portions that are parallel to the sheet conveyance direction are held down such that both side portions do not rise upward.